1. Field of the Invention
The present invention relates to a discharge reactor for producing silent discharge and/or creeping discharge so that a substance will undergo reaction in the discharge space.
2. Description of Prior Art
Discharge reactors of the type contemplated by the present invention are typically used as an ozonizer and an ionizer, most commonly as an ozonizer. An ozonizer and other apparatus in which a dielectric is provided between a high voltage electrode and a grounded electrode and silent discharge or creeping discharge is caused to occur between the dielectric and the high voltage electrode and/or grounded electrode are available in various models and constructions as described below.
FIGS. 1-5 show diagrammatically the constructions of five typical discharge reactors that have heretofore been proposed or commercialized. In FIGS. 1-5, the reference numeral 2 denotes a dielectric, 3 is a high voltage electrode and 4 is a grounded electrode. Shown by 5 in FIGS. 1, 2, 4 and 5 is an insulator that insulates electrically the high voltage electrode 3 and the grounded electrode 4 with the dielectric 2 being interposed. The insulator 5 also works as a sealant to isolate the discharging area from the ambient atmosphere to form a discharge space 1. Shown by 6 in FIG. 3 is an insulating/sealing wall that established electric insulation from the ambient atmosphere to form the discharge space 1. Shown by 7 in FIGS. 1-5 is a high voltage ac power supply that is connected to the high voltage electrode 3 and the grounded electrode 4 and which applies high ac voltage to produce discharge in the space 1.
FIG. 1 shows a discharge reactor in which two dielectrics 2 are provided on the surfaces of the electrodes 3 and 4 for causing silent discharge to occur between the dielectrics 2 in the discharge space 1. In the apparatus shown in FIG. 1, neither the high voltage electrode 3 nor the grounded electrode 4 is exposed in the discharge space 1.
The discharge reactor shown in FIG. 2 has basically the same construction as the apparatus shown in FIG. 1 except that dielectric 2 is provided only on the surface of grounded electrode 4 and that high voltage electrode 3 has its surface exposed in the discharge space 1. In the apparatus shown in FIG. 2, silent discharge is generated in the gap between high voltage electrode 3 and dielectric 2.
The discharge reactor shown in FIG. 4 has basically the same construction as the apparatus shown in FIG. 2 except that dielectric 2 is provided only on the surface of high voltage electrode 3 and that ridges 4-1 are provided on grounded electrode 4 in order to ensure that a high density discharge is produced at a comparatively low voltage. The minimum uniform gap G between the surface of the dielectric 2 and the tops of ridges 4-1 on the grounded electrode 4 is usually not wider than 0.5 mm. Silent discharge will occur predominantly in the apparatus shown in FIG. 4 but since creeping discharge occurs at the tops of ridges 4-1, a combination of two types of discharge, so called silent and creeping discharges, occur in the apparatus.
In the discharge reactor shown in FIG. 5, dielectric 2 is provided between high voltage electrode 3 and grounded electrode 4 and discharge space 1 is provided not only between the high voltage electrode 3 and dielectric 2 but also between the dielectric 2 and the grounded electrode 4. Silent discharge occurs in each of the two discharge spaces 1.
In the discharge reactor shown in FIG. 3, dielectric 2 is provided on surface of the grounded electrode 4 whereas a high voltage electrode 3 smaller than the grounded electrode 4 and dielectric 2 is provided on top of the dielectric 2, with discharge space 1 being provided above the high voltage electrode 3. Creeping discharge occurs in the space 1 between high voltage electrode 3 and dielectric 2. The apparatus may be modified by interchanging the positions of high voltage electrode 3 and grounded electrode 4.
The discharge reactors shown in FIGS. 1-5 are basically of a flat plate type but they may be of a cylindrical type or the positions of the grounded electrode and the high voltage electrode may be interchanged. In addition, various modifications may be made to the geometry of electrodes, their layout, etc.
If, given a discharge reactor of the same type and dimensions (with a constant area of discharge), one wants to improve its performance, say, the yield and concentration of ozone, it is necessary to generate a high-density silent discharge or creeping discharge (the two types of discharge are hereunder collectively referred to as "ozonizer discharge") by increasing the power for discharge (power input per unit area of discharge). While various electrical factors are involved, the capacitance contributed to discharge of the apparatus at issue is particularly important and for the purpose of generating a ozonizer discharge of high density, the higher the capacitance C of the dielectric used, the better.
The capacitance C of a dielectric per unit area is expressed by the following equation: EQU C=.EPSILON./t
where .epsilon. is the dielectric constant of the dielectric and t is the thickness of the dielectric.
As one can see from the equation, the dielectric constant of the dielectric must be increased or the thickness t of the dielectric must be reduced in order to increase its capacitance C. The dielectric constant .epsilon. is inherent in a given dielectric material and ceramics commonly used today show .epsilon. values of at least 9 and there are very few practical substances that excel them. Therefore, the current method used to increase the power for discharge is reducing the thickness t of the dielectric but, then, the withstand voltage or the strength of the dielectric will decrease, or difficulty is encountered in the manufacturing process or a further reduction in thickness will occur on account of a sputtering effect due to discharge; all of these factors contribute to a shorter service life of the dielectric. For practical purposes, a thickness of 0.1-1 mm will suffice but in general, a minimum thickness of 0.5 mm is necessary.
In the discharge reactor shown in FIG. 1, two dielectrics 2 are provided on the surfaces of the two electrodes, making the thickness t twice as large. Hence, the capacitance of the dielectric 2 is halved and this is disadvantageous for the purpose of increasing the power for discharge. As a result, it is not easy to generate the ozonizer discharge of high density necessary for increasing the concentration and yield of ozone. However, even this arrangement can be used to fabricate a commercial discharge reactor if the reactants are extremely corrosive and, hence, are hazardous to metallic electrodes.
Further, discharge reactors having the constructions shown in FIGS. 1-5 have heretofore used glass as the material of dielectric 2 in order to meet the requirements for good electrical insulation and high withstand voltage, as well as the ability to withstand the erosive action of a reaction product such as ozone. Today, ceramics having comparatively high dielectric constants are used in reactors with a view to improving ozone generation and increasing the concentration of the product ozone.
As mentioned above, the ceramics used as the material of dielectric 2 are selected from among those of comparatively high dielectric constants which have good electrical insulation, high withstand voltage and high corrosion resistance. Such ceramics are typically used as independent fired plates or, alternatively, they are baked or plasma sprayed onto the high voltage electrode 3 or grounded electrode 4. In whichever case, the formation of materials defects such as vacancies (e.g., pinholes) and voids in the bulk or surface of the dielectric 2 is unavoidable. Furthermore, the presence of impurities or gaseous components in the raw materials of ceramics and their ingredients is unavoidable, as is the introduction of such impurities or gaseous components during the mixing, shaping and firing steps. As a result, the dielectric 2 contains significant amounts of impurities.
Even if the pinholes in the dielectric 2 are small (&lt;tens .mu.m) under ozonizer discharge, abnormal discharge or void discharge (i.e., a discharge that develops in small cavities in a dielectric) will occur in the neighborhood of those small pinholes, leading to deterioration or failure of the dielectric 2. In addition, highly concentrated ozone has great oxidizing power and, hence, erosive action; furthermore, the high density discharge that has become possible as a result of improvement in the performance in ozone generation accelerates the sputtering effect, whereby the surface of the ceramic material (dielectric 2) is scraped and deleterious void discharge is accelerated.
As a consequence, the life of the dielectric 2 is shortened and the performance in ozone generation is lowered. Furthermore, the eroded or scraped dielectric 2 will release deleterious ingredients, impurities, harmful gases, etc. from the ceramic material, thereby contaminating the reaction product (ozone gas). To avoid this problem, one of the following treatments has been applied to the surface of the dielectric 2 (ceramic material):
(1) applying a highly flowable, insulating glaze onto the surface of the dielectric 2 and glazing it so that materials defects through the surface are covered to enhance the strength of the dielectric, thereby controlling the occurrence of abnormal discharge; PA1 (2) forming an extremely thin film of high-purity alumina (Al.sub.2 O.sub.3) or quartz (SiO.sub.2) having high resistance to sputtering by a suitable technique such as CVD, sputtering or ion plating.
The first approach enables the formation of a thick coating and, hence, is very effective for the purpose of correcting materials defects and controlling the occurrence of abnormal discharge. On the other hand, the presence of impurities in the applied coating after firing is unavoidable and they will contribute to surface toughening on account of the sputtering effect caused by discharge; therefore, in the case where the reaction product (e.g., ozone) is required to be highly pure, such impurities will be released from the coating to contaminate the reaction product.
To form a high-purity and damage free film by the second approach, the film thickness must not exceed a few microns. However, the film will, in any case shed off or be otherwise damaged in less than 200 hours, thus totally failing to serve the purpose. Furthermore, the heretofore employed techniques such as CVD, sputtering and ion plating are complicated and time consuming and consequently expensive.
With recent improvements in the performance of ozonizers, the use of ozone in during semiconductor fabrication is increasing and there is a growing demand for the development of an ozonizer that has a long service life, that is highly reliable and that is capable of producing an ozone gas which is highly concentrated and free of impurities.
The discharge reactors of the types shown in FIGS. 2-5 use only one dielectric 2 and, hence, they are advantageous from the viewpoint of the capacitance of dielectric 2. However, as can be seen from FIGS. 2-5, those reactors have either one or both of the electrodes exposed in the discharge space 1 and a discharge is generated between the surface of the exposed electrode (which may be either the high voltage electrode 3 or the grounded electrode 4 or both) and the dielectric 2. Under the circumstances, the electrode material must be such that it can withstand the erosive action of ozonizer discharge and ozone; if the density of discharge is comparatively low, an austenitic stainless steel is commonly used as the electrode material and if the density of discharge is high, tungsten or titanium is often used as the electrode material.
Japanese Patent Public Disclosure (Laid-Open) No. 245,236/1990 filed by the same assign as that of the subject application discloses, as an electrode material(s), Al (purity: 98.S wt % or less) or Al-alloy having at least the area of the discharge space which is to be exposed for discharging is further coated with an anodic oxidation film. Alternatively, it discloses as an electrode material(s) Ti or Ti-alloy having at least the area of the discharge space which is to be exposed for discharging is further coated with an anodic oxidation film or a hot anodic oxidation film. When the procedure of Example in this application was followed using alumina ceramics (purity: 96 wt %) as a dielectric material, ozone was generated in a concentration of about 4-5 vol % with alkali metals, alkaline-earth metals and heavy metals being present in several hundred ppt.
Further, as a material used for a high voltage electrode and/or a grounded electrode which is adhered to the surface of a dielectric without coming into contact with the discharge zone, a film of Ag, Ag-Pd alloy, Au, or Mo-Mn alloy has been conventionally used in a discharge reactor.
With reference to FIGS. 2-5, the surface of either high voltage electrode 3 or the grounded electrode 4 or both are exposed to a mixed field of concentrated ozone which has great oxidative power and high-density ion or plasma and, hence, the electrodes will be consumed, causing the electrode material to enter the reaction product (ozone gas), typically as an oxide, whereby not only is the reaction product contaminated but also the electrode material is redeposited on the electrode or deposited on the dielectric 2, leading to deterioration in performance as a result of the synergistic effect in combination with the electrode consumption. Furthermore, the electrode material in oxide form will contaminate the inner surface of the discharge space 1. This phenomenon tends to become worse as the density of discharge increases. Depending on the electrode material, substances that are deleterious to the user of ozone gas may be released as sources of contamination.
With the recent improvement in the performance of ozonizers, their application to the process of semiconductor fabrication is increasing and there is a growing demand for the development of an ozonizer that is capable of producing "clean" ozone gas which is highly concentrated and substantially free of impurities. The process of semiconductor fabrication particularly hates contamination by alkali metals, alkaline-earth metals and heavy metals; however, the dielectric materials and the electrode materials to be used in ozonizers contain relatively large amounts of alkali metals and alkaline-earth metals such as Na, K, and Mg, and heavy metals such as Fe, Cu, Cr and Ni and, therefore, the consumption of the dielectric and the electrodes will result in the contamination of the ozone gas generated, making it totally unfit for use. Al impurity will not result in any great adverse effect in the process of semiconductor fabrication compared to the alkali metals, the alkaline-earth metals and the heavy metals. However, amounts of Al in an ozone gas generated is analyzed in view of checking erosion of Al-base dielectrics and electrodes.